Caddisfly egg mass morphology mediates egg predation: potential costs to individuals and populations

Abstract

Summary Egg predation is seldom considered in life‐history studies of freshwater insects, but could be an important source of mortality with potential to limit population numbers. Costs of egg predation to prey can be considered at two levels: (i) fitness costs to individuals via reduced reproductive output; and (ii) population costs via reduced recruitment of benthic larvae. Larvae of Orthotrichia armata (Trichoptera: Hydroptilidae) feed on the egg masses of caddisflies in the Little River, central Victoria, Australia, but the small size of O. armata suggests that they may be unable to penetrate egg masses that are covered with a thick layer of spumaline jelly. We predicted that predation by O. armata would be more severe for egg masses with thinner spumaline. With field surveys throughout the peak caddisfly oviposition period (summer), we compared predation by O. armata on the egg masses of nine caddisfly taxa. Egg masses were categorised, according to the thicknesses of spumaline, into bulbous (thickly jellied), thinly jellied and jelly‐free morphotypes, with three species per morphotype. Bulbous egg masses were rarely consumed. Thinly jellied egg masses comprised just 4–13% of all egg masses in each survey, but attracted 52–93% of all egg predators. Jelly‐free egg masses were readily consumed, but O. armata were disproportionately scarce on them. In a second survey, we compared the proportions of eggs that were consumed from egg masses with small and large clutch sizes, by monitoring the thinly jellied egg masses of Taschorema sp. (598 ± 95 eggs per mass) and Ethochorema turbidum (1195 ± 192 eggs per mass) to eclosion. More than 25% of Taschorema sp. egg masses were entirely consumed, causing complete reproductive failure of parent females (fitness costs to individuals). Consumption rates were < 25% for most egg masses of E. turbidum. Orthotrichia armata consumed 48% of all Taschorema sp. eggs and 20% of E. turbidum eggs, suggesting that egg predation may substantially curb larval recruitment (costs to populations), particularly for species with small clutch sizes. Our results demonstrate that egg mortality varies for aquatic insect species that package their eggs in different ways. Combined with recent data from the same system, our results suggest egg mass morphology–mortality trade‐offs, with bulbous egg masses particularly vulnerable to flow forces (shear) and thinly jellied/jelly‐free egg masses particularly vulnerable to predation. Given the wide array of aquatic invertebrate species that lay single egg masses on hard substrata, it is feasible that this is a common trade‐off that warrants more research on the impacts of egg predation on aquatic species.

Egg masses of some stream‐dwelling caddisflies (Trichoptera: Hydrobiosidae) from Victoria, Australia

“…facilitate empirical estimates of recruitment for studies on population dynamics. Egg mortality of these Hydrobiosidae appears to be low, apart from some predation by other aquatic insects (Reich 2004;Bovill et al 2015;Lancaster pers. species coexistence mechanisms, community assembly) are possible because multiple species of Hydrobiosidae often co-occur in the same river.…”

“…Traditionally, freshwater ecological research has focussed on the aquatic juvenile stages and predominantly late instar larvae, but estimating the factors that drive population dynamics requires examination of multiple life stages, including the often-neglected stages of eggs, pupae and adults. The success of such studies, however, depends on understanding the environmental factors that influence oviposition behaviours, and hence the spatial and temporal distribution of eggs within water bodies (Hoffmann & Resh 2003;Lancaster et al 2003;Reich & Downes 2003a, 2003bEncalada & Peckarsky 2007;Lancaster et al 2010b;Reich et al 2011;Storey et al 2016), and on understanding the factors that can influence successful hatching of eggs and recruitment of juveniles into the population (Siva-Jothy et al 1995;Purcell et al 2008;Bovill et al 2013;Bovill et al 2015;. Pupae are rarely considered by freshwater ecologists but high, density-dependent mortality of pupae can regulate some populations (Marchant & Hehir 1999).…”

“…There are over 15 genera and over 60 species of Australian Hydrobiosidae, occurring primarily in eastern Australia, with the highest species richness in south-eastern Australia and Tasmania (Dean 1997). Subsequent ecological research on these same genera has provided further insight into the consequences of oviposition behaviour for the spatial arrangement of egg masses (Lancaster et al 2003;Reich et al 2011), and into the effects of flow and egg predation on hatching success (Bovill et al 2013;Bovill et al 2015). an 'emergent rock'), climb down the side of the rock and lay their eggs in a single, plaque-shaped mass covered in spumaline and attached to the underside of the rock.…”

Variations in fecundity over catchment scales: Implications for caddisfly populations spanning a thermal gradient

“…All three species oviposit exclusively on emergent rocks (rocks that protrude from the water) making it easy to locate egg masses (Reich, ). Each female lays her entire clutch of eggs in a single egg mass and, in the absence of hydraulic disturbance, the eggs of these species hatch with near 100% success in the field (Bovill, Downes, & Lancaster, , ; Reich, ), meaning that estimates of egg numbers are likely to represent recruitment numbers of neonate larvae that enter the benthos. Based on measures of forewing length (a surrogate for body size), average body size of females is similar for all species ( A. obliquum 8–9.5 mm; A. gisbum 8.5–10 mm; U. rubiconum 8–10 mm; Neboiss, 1986).…”

Evolution of egg coats: linking molecular biology and ecology

“…Salthe ; Pechenik ; Podolsky ; Bovill et al . ). Here, we emphasize the need for integrative studies (combining molecular approaches with ecological studies) to gain insight into the multifarious role of egg coats and, in particular, the need to understand intraspecific variation (i.e.…”